I decided to use the 'built-in' nodes browser, and instead use a context menu action to select whether or not to generate roads on the selected spline. The float window will only show some properties for the currently selected spline.

I got a context menu tool to work, but it should only be visible on spline objects. Can I determine the type of object inside queryLayout()?

Also, should this topic be moved because it is no longer about ZModeler 1?